This invention relates to an apparatus for multiplex recording and reproduction of audio and video signals, and more particularly to a recording apparatus suitable for multiplex recording of audio signals and color video signals by use of a magnetic recording/reproduction apparatus such as video tape recorder.
In video tape recorders in general, a video signal is recorded by a rotary head so as to form an oblique track on a magnetic tape, while an audio signal is recorded by a fixed head on a track disposed in the longitudinal direction of the tape at a portion separate from the oblique track (generally, at the edge portion of the tape), as is well known in the art. In a home video tape recorder, a slow running speed is selected for the magnetic tape in order to increase the recording density and to facilitate long time recording. In an NTSC video tape recorder, for example, the tape running speed is prescribed to be about 33 mm/sec in the standard mode and about 11 mm/sec in the triple recording mode. This tape running speed is lower than the tape running speed of an audio tape recorder using a compact cassette. For this reason, the video tape recorder can not provide a sufficiently satisfactory frequency range for the recording and reproduction of audio signals for which the tone quality is of particular importance, particularly in the triple recording mode.
A method of recording and reproducing audio signals by use of a rotary head in the same way as video signals has therefore been proposed to eliminate the problem described above. In the home video tape recorder, the color television signal is recorded by a common rotary head on the magnetic tape as an addition signal of a signal obtained by frequency-modulating a suitable carrier by a luminance signal and a carrier color signal which is down-converted below the band of the frequency-modulated carrier signal described above. Recording of the audio signal in accordance with this proposal is made in a superposed manner by the rotary head on the same recording track with the video signal as a frequency-modulated signal with its band set between the band of the luminance signal generated as the frequency-modulated signal and the band of the carrier color signal converted to a low frequency band. As one of the means for the multiplex recording of the audio and video signals, it has been proposed to use rotary heads that are individually disposed for the audio and video signals in such an arrangement that these heads can trace the same track on the magnetic tape. In this case, the frequency-modulated audio signal of the low frequency band is first recorded, and the frequency-modulated luminance signal having a high frequency is then recorded. According to this method, the signal having a low frequency magnetizes the deep portion of the magnetic layer of the magnetic tape, whereas the signal of a high frequency magnetizes only the shallow portion of the magnetic layer of the tape. Therefore, the shallow portion of the recording magnetization by the audio signals having the low frequency (in the form of the frequency-modulated signal, of course) will be erased by the subsequent recording of the frequency-modulated luminance signals, but the deep portion remains unerased. As a result, two kind of signals can be recorded superposed on one track at the deep portion of the magnetic layer and at its shallow portion close to the surface.
This multiplex recording method of the video and audio signals is effective where no margin for multiplexing them as a frequency multiplex signal exists between the frequency band of the frequency-modulated luminance signal and that of the carrier color signal converted to the low frequency band. When the color video signal is recorded in the VHS system video tape recorder, for example, it is stipulated to convert the color video signal so as to attain the frequency spectrum such as shown in FIG. 1. In the case of the luminance signal of the NTSC signal, the carrier signal is frequency-modulated so that the leading edge of a sync signal is 3.4 MHz and the white peak level if 4.4. MHz, and a frequency distribution such as represented by reference numeral 4 is provided. In the case of the carrier color signal, the carrier frequency is converted to about 629 KHz, and a frequency distribution having a band width of about 1 MHz, such as represented by reference numeral 5 in the drawing, is provided. Thus, there is no margin between the distribution 4 of the luminance signal and that of the carrier color signal. For the spectrum of the video signals for recording, the audio signal converted to the frequency-modulated signal is set within the frequency range represented by reference numeral 6 in the drawing, and is recorded and reproduced by the separate rotary head.
Some of the inventors of the present invention previously proposed an apparatus for superposedly recording the audio signal on the track of the video signal in U.S. patent application Ser. No. 575,665 dated Jan. 31, 1984. The previous application proposes the arrangement of each head in order to minimize the interference due to cross talk from the adjacent tracks and the interference between the video signal and the audio signal.
In the home video tape recorders in general, the optimum recording current at which the reproduction output becomes maxium within a practically suitable range tends to decrease with a higher signal frequency. Therefore, predetermined frequency characteristics are set to a recording amplifier. In the VHS system, for example, the characteristics are set so that a 1 MHz recording current is about 6 dB when the recording current of a 3.4 MHz signal is 0 dB.
Therefore, the carrier corresponding to the leading edge of the sync signal becomes greater than the carrier corresponding to the white peak in the recording current of the video signal. As a result, the frequency-modulated audio signal, that has been recorded before the video signal, is not uniformly erased by recording of the subsequent frequency-modulated luminance signal when the former is somewhat erased by the latter, so that the extent of the erasure varies depending upon the content of the video signal. In other words, the extent of the erasure becomes maxium at the sync signal and minimum at the white peak. Therefore, if the luminance signal of the video signal, such as shown in FIG. 2a, is recorded, the amplitude of the reproduction output of the frequency-modulated audio signal is amplitude-modulated corresponding thereto, as shown in FIG. 2b. FIG. 2b shows the envelope of the reproduction output of the signal which is frequency-modulated by the audio signal, and the amplitude is lowest at the portion 20 corresponding to the sync signal and greatest at the portion 21 corresponding to the white peak. Thus, though frequency-modulated, the origial audio signal can be obtained by demodulation because it is fundamentally a frequency-modulated signal. The influence of the amplitude modulation generated by the erasing action of the recording of the luminance signal is not so great as to cause distortion in the demodulated signal waveform. In the case of the reproduction signal shown in FIG. 2b, however, the carrier-to-noise ratio C/N of the portion 20 corresponding to the sync signal is deteriorated more than the C/N of the portion 21 corresponding to the white peak. (Since the audio signal in this case is one obtained by frequency-modulating the carrier, the ratio C/N is used so as to distinguish it from the signal-to-noise ratio S/N.) As a result, when this signal is demodulated, the noise of the portion corresponding to the portion 20 relatively increases much more than that of the portion 21 and eventually, the S/N of the portion 20 becomes worse than that of the portion 21. This means that in the demodulated audio signal, noise increases periodically in synchronism with the sync signal of the video signal. Particularly, the increase of the noise, that appears in synchronism with the vertical sync signal, is very noticable to the listener and is unpleasant to the ear, even though it is not great, because the fundamental repeating frequency is as low as 60 Hz, for example, and because it is the noise that did not exist originally.